Ablative materials are employed as part of the thermal protection system (TPS) on space vehicles, such as in the formation of heat shields and insulating coatings. Several ablative foams presently are used in the TPS of the United States space shuttle and, in particular, are used in the TPS of the external tank (ET). Historically, these materials have been selected from or modified from commercially devised foams. Until now, no single foam has been found that meets both engineering and manufacturing constraints for all ET applications, so several different foams must be used.
A major reason that several foams have been necessary is that most foams are designed to meet the requirements of the market's major users, which are the construction and refrigeration industries. Common commercial urethane and isocyanurate foams are intended for use as roof top insulation, piping insulation, refrigerated container insulation, and the like. These commercial uses financially and technically dictate the types of materials being manufactured by the compounders and processors. Thus, the design criteria for most foams are based on pre-established industrial standards. The dominating commercial design factors are cost per pound, insulation value, strength, and flammability in place. In general, commercial foams are classified according to their flammability and smoke generation characteristics, most commonly using the ASTM E-84 test, which compares the material to red oak. In this test, red oak arbitrarily is given a value of 100 both for smoke generation and for flammability. Class 1 foams, which have a 25 rating or less, generally are the only candidates that are screened for usage on the shuttle's external tank. However, without modification even these Class 1 foams have little chance of meeting the engineering criteria for use on the external tank of a space shuttle, since they are designed to be cost-competitive in broad commercial markets.
As a practical matter, the selection of new foams for space shuttle use is further constrained by the established facilities, production methods, and the like that have been designed around the currently used foam's process parameters. For example, when a new foam is selected, it is significantly more efficient and reliable to continue to use the existing facilities and tooling required to apply the currently used foam. Therefore, any backup or replacement foam must be processable within constraints no tighter than those of the current foam if major tooling costs and facilities costs are to be avoided.
The particular needs of the aerospace industry for ablative foams seldom are addressed by the commercial compounders since the quantity of materials used is small. Thus, in the past it has been necessary to start with a commercial foam and work with the compounder to make minor modifications so that the foam meets certain minimum criteria. Such modifications can be quite challenging due to the nature and complexity of the material's chemistry. Thus, in spite of well known specific engineering criteria established for each type of foam used on the external tank, the ET foam needs continue to be filled by modified commercial foams.
The space shuttle and its external tank present specific harsh conditions that must be met by any chosen ablative foam material. Similar requirements are associated with other cryogenically fueled boosters, as well. The harsh conditions which are encountered can be best understood by reference to some of the specific and most difficult mission requirements.
One such requirement is that the foam be strain-compatible with an aluminum substrate at cryogenic temperature. For example, foams that become brittle at minus 423.degree. F. could fail by fracturing or delaminating from the external tank.
Another requirement is that the foam have satisfactory strength at elevated temperature to prevent explosive loss of chunks. Such loss may occur due to internal cell pressure, when the foam is heated to 300.degree. F. from heat sources internal and external to the propellant tanks. Compounding the situation is the near-vacuum external pressure that is encountered during portions of the vehicle's flight.
Additionally, a satisfactory foam must form a dimensionally stable char as a result of thermal decomposition. A part of the foam's thermal protective function is achieved when the surface of the foam forms a char. As long as the char maintains a continuous surface and does not, for example, form wide and deep cracks due to excess char shrinkages, the char achieves the protective function. In addition, the foam must be self-extinguishing in air.
A satisfactory foam must also have low friability. It is generally known that highly three-dimensionally crosslinked isocyanurate foams have been produced in the prior art. These foams may have excellent high temperature properties and yield sturdy chars. However, they are very brittle and damage-prone, and therefore are of little practical value.
At least three different foams presently are used in the TPS for the external tank. The first, a high performance isocyanurate-type spray foam (CPR 488 and NCFI-22-65), is used for major acreage, high-performance applications. This foam is manufactured under highly restrictive processes to assure the material's integrity and performance for flight environments. A second foam (BX-250) is a more generic urethane spray foam, used for ice and frost protection and genera closeouts. This second type is relatively more easily processed and also meets certain design and flight criteria. A third type of foam (PDL-4034) is a pour foam utilized for manufacturing molded parts and general repair of complex areas.
It would be desirable to have a single foam capable of meeting the engineering criteria for all foam uses on the external tank, thus simplifying logistics. Also, by its broad performance range, such single foam likely would provide improved performance in some or all application areas by meeting or exceeding the performance capabilities of the presently used materials.
A number of patents are known to relate to either ablative compositions or to general foam compositions. U.S. Pat. No. 4,077,921 to Sharpe, et al. teaches a sprayable, low density ablative composition employing epoxy-modified polyurethane resins carrying microballoons. This patent is notable because the disclosed composition is for use as an ablator on the solid fuel rockets of the space shuttle. However, the composition itself has little similarity to the present invention.
Other patents disclose isocyanate or isocyanurate foams, which are of the general type produced in the present invention. Polyisocyanurate foams, which are known to have high char resistance and low smoke values by commercial standards, are formed by trimerizing polyisocyanates. The resulting foams, like urethane foams, also have low thermal conductivities. However, commercial versions are known to be brittle and highly friable. These undesirable characteristics have been modified by adding to the isocyanate a urethane polyol, such as a glycol, although the resulting modified foam has less favorable burning and smoke characteristics.
The foams of other known patents are believed to be directed to the major commercial uses, such as construction and refrigeration. For the reasons set out above, these foams are most certainly inadequate for the performance criteria of TPS foams on the external tank of the space shuttle and other cryogenically fueled boosters. However, since in aerospace foams the common commercial criteria are desired, although to a superior degree, these patents are noted for teaching the direction of the art.
One such patent is U.S. Pat. No. 3,647,724 to Doerge, which discloses a polyurethane foam composition with reduced smoke levels, achieved by addition of chlorendic acid. The foam is formed by reaction of an organic polyisocyanate with an active hydrogen-containing material, and a phosphorous fire retardant. The active hydrogen-containing material may be an organic polyol, preferably a polyether polyol having hydroxyl value between 200 and 800. The organic polyisocyanate may be polymeric diphenylmethane-diisocyanate (polymeric MDI), as well as other polymeric polyisocyanates having a functionality greater than 2.0. The fire retardant phosphorus may be contributed by either a reactive or non-reactive compound and supplies from 0.5 to 2.0 percent phosphorus. Optionally, about 0.10 to about 3 weight percent silicon base surfactant may be used in less dense foams and may be omitted entirely in foams weighing 5 to 6 pounds per cubic foot. The foam is formed in conventional manner by use of a blowing agent and a catalyst, such as a tertiary amine or an organic salt of tin. The resulting foam is stated to be self-extinguishing under a commercial ASTM standard.
U.S. Pat. No. 4,426,461 to Smith describes the preparation of a diisocyanurate foam by reaction of a low-functionality methylene diisocyanate with a polyol and a silicone surfactant in the presence of a fluorocarbon blowing agent and a catalyst, such as a quaternary ammonium salt. The resulting product is stated to be suited for use in the construction and insulation field.
U.S. Pat. No. 3,384,599 to Omietanski et al. teaches preparation of an organic polyol modified with organosiloxane groups. The dimers and trimers of isocyanates are identified as suitable reactants with polyol-siloxane compositions. The resulting polyurethane foams are stated to have uniform cell structures.
U.S. Pat. No. 3,399,247 to Windemuth et al. teaches a method of producing polyurethane foam by reaction of a polyether with an organic polyisocyanate and an organosiloxane. The resulting foams are stated to have utility as upholstery, insulation, sponges and the like.
U.S. Pat. No. 3,642,646 to Marcus describes the use of carboxy-bearing adduct polyols, formed by reaction of a polyol and the anhydride of a polyfunctional aromatic carboxylic acid or chlorendic anhydride. The aromatic carboxylic acid anhydride may include tetrachlorophthalic anhydride or tetrabromophthalic anhydride. The adduct polyols are reacted with a polyphenylisocyanate, a fluorocarbon blowing agent, and a catalyst to produce a foam, which the disclosure states will meet commercial standards for fire-resistant foams.
U.S. Pat. No. 4,133,781 to Ashida et al. discloses a polyisocyanurate foam that is stated to have low smoke generation, low friability, high heat-resistance and high flame-retardance, and no bursting property. This result is achieved by adding to the polyisocyanurate foam from 2 to 30 weight percent of an organosilicate having hydroxypolyoxyalkylene groups. As expected, the performance of this foam is measured against ASTM and other common commercial standards.
U.S. Pat. No. 3,388,101 to Wismer et al. discloses the formation of polyurethanes by combination of a polyisocyanate and an organosilane.
Therefore, it would be desirable to develop an ablative foam suited to the needs of the aerospace industry, yet that is suited for use with established facilities, production methods, and tooling.
To achieve the foregoing and other objects and in accordance with the purpose of the present invention, as embodied and broadly described herein, the ablative foam and method of manufacture of this invention may comprise the following.